1. Field of the Invention
The present invention relates to a thin film electroluminescence element which emits light of high luminance by applying an AC voltage thereto, and more particularly to an electroluminescence element with a luminance that can be easily controlled.
2. Description of the Prior Art
A thin film electroluminescence(hereinafter called EL) element is composed of a semiconductor luminous layer formed by doping an impurity such as Mn, Cu, TbF.sub.3 and the like into a crystal body of Zns, ZnSe or the like. The above impurity acts as an active material for forming a luminescence center. By applying an AC field to the semiconductor luminous layer, EL luminescence is obtained.
It is well known to use a thin film EL element having a double layered insulation construction obtained by covering both surfaces of the luminous layer with dielectric films made of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, TiO.sub.2 or the like for increasing the luminance of light emitted thereby.
One example of this thin film EL element having such a double layered insulation construction is shown in FIG. 5.
In FIG. 5, on a glass base plate 1 are formed a transparent electrode 2 made of In.sub.2 O.sub.3, SnO.sub.2 or the like and a first dielectric film layer 3 (hereinafter called dielectric body layer) made of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, TiO.sub.2 or the like by sputtering or electron beam evaporating.
On the first dielectric body layer 3, is formed a ZnS luminous body film layer 4 (hereinafter called a luminous body layer) by electron beam evaporating. In this case, a predetermined density of Mn is mixed into the luminous body layer 4 for forming a luminescence center. On the ZnS luminous body layer 4 is piled a second dielectric body layer 5 made of a material similar to that of the first dielectric body layer 3. Then, on the second dielectric body layer 5 is formed a rear electrode 6 made of Al or the like by an evaporating method. An AC power source 7 is connected between the transparent electrode 2 and the rear electrode 6.
When an AC voltage is applied between the transparent electrode 2 and the rear electrode 6, the above described AC voltage appears between the dielectric body layers 3 and 5 on both sides of the ZnS luminous body layer 4 thereby to produce an electric field within the ZnS luminous body layer 4.
By virtue of the electric field within the ZnS luminous body layer 4, electrons obtain a sufficiently large energy, and these electrons (hereinafter called hot electrons) cause the Mn luminescence center to emit orange light when the luminescence center returns to its normal state.
The thin film EL element having the above described structure can be used as a thin display device in output display terminals of a computer or in other various display devices.
When this EL element is used as the display device, a large number of parallel transparent electrodes 2 and a large number of parallel rear electrodes 6 are formed like bands, respectively, and are arranged so as to cross at right angles into a matrix electrode construction. One crossing point of one transparent electrode 2 and one rear electrode 6 composes one picture element.
In the display device using the thin film EL element, the operation voltage thereof is low as compared with the conventional cathode-ray-tube and the weight and the strength thereof are excellent as compared with a plasma display panel which is a similar flat type display device.
Furthermore, the display device using the thin film EL element has a wide operable temperature range, a high response speed and other many advantages as compared with a liquid crystal panel.
In addition, the operational life of the display device using the thin film EL element is long because of the solid state matrix panel.
However, in the conventional thin film EL element, the luminance thereof is rapidly increased in the vicinity of V.sub.th (the threshold of luminance) as shown by the line X of FIG. 3. This results in it being difficult to linearly control the luminance by adjusting the voltage becoming applied thereto.